Updating a field from another DAC

@development93

In addition to what @ddunn  has mentioned, you can use FieldSelecting Event to retrieve the desired value from a DAC and show it in your custom/standard field of the current DAC. For example, below I have added a custom field (UsrClassName) to my GLTran (Lines of GL Transaction Screen) and shown the Account Class from the Account table for it. If you have the fields and the reference/current table in a DAC Extension you can use dacAlias.GetExtension<>() (also I have put in my code for your reference). You need to replace the pieces as per your need to make it.

// ********************* Display GL Account Class in GLTran *********************

using HCL;
using PX.Data;

namespace PX.Objects.GL
{
    // DAC Ext
    [PXNonInstantiatedExtension]
    public sealed class USRGLTranExt : PXCacheExtension<PX.Objects.GL.GLTran>
    {
        public static bool IsActive() => true;

        #region UsrClassName
        public abstract class usrClassName : PX.Data.BQL.BqlString.Field<usrClassName> { }
        [PXString(60, IsUnicode = true)]
        [PXUIField(DisplayName = "Account Class", Enabled = false, Visible = true)]
        public string UsrClassName { get; set; }
        #endregion
    }

    // Graph Ext
    public class USRJournalEntryExt : PXGraphExtension<PX.Objects.GL.JournalEntry>
    {
        public static bool IsActive() => true;

        protected void _(Events.FieldSelecting<GLTran, USRGLTranExt.usrClassName> e)
        {
            if (e.Row == null) return;

            e.ReturnValue = "";

            if (e.Row.AccountID != null)
            {
                Account account = PXSelect<Account,
                    Where<Account.accountID, Equal<Required<Account.accountID>>>>.Select(Base, e.Row.AccountID);

                if (account != null && account.AccountClassID != null)
                {
                    //Get any DAC Extension using below 1 line of code
                    MyAccountExt accountExt = accountExt.GetExtension<MyAccountExt>();

                    AccountClass accountClass = PXSelect<AccountClass,
                        Where<AccountClass.accountClassID, Equal<Required<AccountClass.accountClassID>>>>.Select(Base, account.AccountClassID);

                    if (accountClass != null)
                    {
                        e.ReturnValue = accountClass.Descr;
                    }
                }
            }
        }
    }
}

//Add the "UsrClassName" to the Transaction Grid (GLTranModuleBatNbr) in GL301000 and set the CommitChanges = True as follows:
//  <px:PXGridColumn DataField = "UsrClassName" Width="220" CommitChanges="True" />